Switched capacitor sine wave generator and keyer

Abstract

The invention relates to a circuit for synthesizing either simple or complex waveforms of the type used in electronic musical instruments, such as electronic organs. In the specific embodiment disclosed herein, the synthesizing circuit is utilized in a rhythm unit for producing a damped sine wave charateristic of certain drum sounds. Opposite polarity waveforms are simultaneously produced by a switched capacitor technique driven by an attack/decay clocking signal and under the control of a keying signal received from a suitable low frequency rhythm clock source. The positive and negative waveforms are alternately selected in order to produce bipolar pulses at the frequency of the desired tone, and these pulses are connected to the input of a switched capacitor filter that modifies the pulses to produce a sine wave signal having an amplitude following that of the desired damped envelope. The individual sine wave cycles are produced by generating a plurality of increments wherein each increment comprises a plurality of discrete amplitude steps. The stepped waveform is then filtered by a second switched capacitor filter that is tracked to the frequency which drives the sine wave generation portion of the circuit. The system is particularly adapted for large scale integration requiring no external capacitors and no external tone generating source. A wide variety of waveforms can be generated by the technique of the present invention, and the waveforms can be keyed from a keyboard, an automatic rhythm unit or other control signal within the instrument.